The present invention concerns processing of integrated circuits and pertains particularly to a laser interferometry endpoint detection with windowless polishing pad for chemical mechanical polishing process.
In a semiconductor manufacturing process, on a semiconductor wafer small electronic devices are formed of separate dies. The semiconductor wafer is processed using materials that are patterned, doped with impurities, or deposited in layers.
It is often necessary to polish a wafer surface to provide a substantially planar surface. This is done, for example, using a chemical-mechanical polishing process. Chemical-mechanical polishing is performed by pressing semiconductor wafer against a rotating polishing pad under controlled chemical, pressure, and temperature conditions. A chemical slurry, such as alumina or silica can be use as a polishing abrasive. The polishing effect on the wafer results in both a chemical and mechanical action.
In situ laser interferometry can be used to determine the end point of a chemical-mechanical polishing process. For example, an optical laser and optical radiation detector are located in a polishing platen. A transparent window is embedded into the platen surface for radiation transmission. The polishing pad has a matching embedded window made of a material that allows transmission of the laser radiation (xe2x80x9cwindowed padxe2x80x9d). The window embedded in the polishing pad is aligned to the window embedded on the platen so that radiation may be transmitted through the platen window and through the pad window. The aligned platen and pad windows can be referred to collectively as an xe2x80x9cendpoint window.
As the platen rotates, the endpoint window encounters the wafer once per rotation, allowing radiation to be reflected from the wafer back through the window to the detector. During polishing of a transparent film that is coated over a substrate (e.g., silicon dioxide over silicon), as the film is removed from the surface, the intensity of the radiation at the detector has a periodicity governed by Equation 1 below:
d=xcex/(2n cos xcex8)
In Equation 1, xe2x80x9cdxe2x80x9d is the distance through the film between peak maxima, xe2x80x9cnxe2x80x9d is the refractive index of the film for the radiation wavelength, xe2x80x9cxcex8xe2x80x9d is the collection angle, and xe2x80x9cxcexxe2x80x9d is the radiation wavelength.
A plot of intensity versus polishing time will yield polishing rate and thickness removal information where the polishing rate is the time derivative of xe2x80x9cdxe2x80x9d in Equation 1 above.
For more information see, for example, U.S. Pat. No. 5,413,941, issued on May 9, 1995 to Daniel A. Koos and Scott Meikle for OPTICAL END POINT DETECTION METHODS IN SEMICONDUCTOR PLANARIZING POLISHING PROCESSES. See also, U.S. Pat. No. 5,609,517, issued on Mar. 11, 1997 to Michael F. Lofaro for COMPOSITE POLISHING PAD.
The Mirra(trademark) chemical mechanical polisher system available from Applied Materials, Inc., having a business address of 2821 Scott Boulevard, Santa Clara, Calif. 95050, utilizes three independent polishing stations. This allows for two-step polishing using two platens for each wafer or three-step polishing using all three platens for each wafer. In the Mirra(trademark) chemical mechanical polisher system, a reactor endpoint detection system is implemented such that each platen and polishing pad utilizes an endpoint window.
When a wafer is polished in a multi-platen chemical-mechanical polishing (CMP) reactor such as the Mirra(trademark) chemical mechanical polisher system, part of the polishing is performed on one platen, and additional polishing is performed on one or more additional platens. Such sequences are used to optimize wafer throughput. The endpoint traces from platens used to polish one wafer may be xe2x80x9cstitchedxe2x80x9d together into a virtually single trace.
CMP may be performed on wafers that simultaneously have different structures with different film thicknesses and even with different transmitting films. For example, when performing CMP for shallow trench isolation (xe2x80x9cSTIxe2x80x9d), some parts of the reflective wafer surface are coated with silicon dioxide (xe2x80x9cSiO2xe2x80x9d) films (n=1.44) while other parts of the reflective wafer surface are coated with a multi-film structure of SiO2 on top of silicon nitride (xe2x80x9cSi3N4xe2x80x9d, n=2.00). These different structures yield different intensity versus polishing time curves that are independent of one another. However, the lateral dimensions of the structures are microscopic and the radiation collection area is several orders of magnitude larger than the structures. Therefore, a plot of radiation intensity versus polishing time is a convoluted average of the contributions of individual different structures present in the sampling area. Rotational and translational movement of the wafer during polishing result in further averaging the collected signal over a larger area of the wafer.
The embedded window in the polishing pad is made of a material that is transparent to the endpoint radiation wavelength. During manufacture, a rectangular hole is cut into the polyurethane polishing pad and the transparent window is glued into place. This configuration, however, has several disadvantages.
For example, the window material is a different surface material than the rest of the polishing pad. During data collection, when the wafer sweeps over the window in the polishing pad, the surface of the wafer is exposed to a polishing pad surface that is different from the remaining polishing pad surface. With abrasive slurry present, polishing pressure remains applied. This can result in deleterious scratching. Non-uniform polishing may also result.
Another disadvantage of using polishing pads with an embedded window is that extra manufacturing steps and materials are required to produce such polishing pads. This makes polishing pads with embedded more expensive than windowless polishing pads.
In accordance with the preferred embodiment of the present invention, a multi-platen chemical-mechanical polishing system is presented. A wafer is polished at a first station. During polishing, an endpoint is detected. The endpoint is detected by generating optical radiation by a first light source. The first optical radiation travels through a translucent area in a surface of a first platen and travels through a first polishing pad. After being reflected by the wafer, the optical radiation returns through the first polishing pad through the translucent area to a first optical radiation detector. The first polishing pad has a uniform surface in that no part of the surface of the first polishing pad includes transparent material through which non-scattered optical radiation originating from the first light source can pass and be detected by the first optical radiation detector. Optical radiation that travels through the first polishing pad and is detected by the first optical radiation detector is haze scattered by inclusions within the first polishing pad. Non-scattered light is absorbed by the first polishing pad.
The wafer is also polished at a second station. During polishing a final endpoint is detected. The final endpoint is detected by generating optical radiation by a second light source. The second optical radiation travels through a translucent area in a surface of a second platen and travels through a window embedded in a second polishing pad. After being reflected by the wafer, the optical radiation returns through the window embedded in the second polishing pad, through the translucent area in the surface of the second platen, to a second optical radiation detector.
In one preferred embodiment, for example, each light source is, an optical laser embedded in a platen. Likewise, the translucent area in a platen can consist of, for example, translucent material embedded into the surface of the platen, a hole in the surface of the platen, or an entire surface of the platen.
In the present invention, a windowless pad is used on the first polishing platen, so that the disadvantages of using windowed pads is minimized. The first half of a xe2x80x9cstitchedxe2x80x9d endpoint trace can be collected with low signal-to-noise ratio on the first platen. Although the signal-to-noise ratio may be too low to precisely detect endpoint, when using multi-platen reactors, the actual endpoint of the process always occurs on the second platen. Therefore, a low signal-to-noise signal may be used to initiate the trace on the first platen without danger of detecting a false endpoint due to noise. Radiation source intensity may be increased to compensate for losses in the first pad, so that the average intensity of detected radiation from the first platen is close to that of the second platen. The second half of the xe2x80x9cstitchedxe2x80x9d endpoint trace is collected from the second platen, where the second half of polishing is performed. A windowed pad is used on the second platen for optimum signal-to-noise ratio and accurate endpoint detection.
The above-described preferred embodiment of the present invention does not completely eliminate the disadvantages introduced by using a polishing pad with a window. However, the above-described preferred embodiment of the present invention does eliminate the deficiencies for at least one polishing station. This can result in the reduction of defects from scratches due to the polishing pad window material, the reduction of non-uniform polishing due to the polishing pad window material, the reduction of added cost of purchasing windowed pads, and the reduction of glazing of the platen windows due to slurry leakage.
Also, when a windowed polishing pad is used, the aqueous medium slurries used for polishing transparent films can be corrosive to the modified polishing pad. For example, in semiconductor manufacturing, inorganic dielectrics are typically polished in basic slurries. In such cases the corrosive medium may attack the glues used to hold the window in place. This can cause leaking behind the polishing pad, causing the quartz window in the platen to become glazed over time. This potential problem is alleviated by the present invention.